The invention relates to lightweight and very strong copper-clad aluminum composite wire (hereinafter referred to as "Cu/Al composite wire") and a method of manufacturing such Cu/Al composite wire, which is used for internal conductors for coaxial cables, electromagnetic shield braided wire for coaxial cables, electric wiring cables for airplanes, automobiles, electric automobiles, portable VTRs and TV sets, voice coards for speakers, magnet wire, and the like.
FIG. 1 is a sectional view of a Cu/Al composite wire. Reference numeral 1 designates an aluminum core; and 2, a copper cladding layer around the aluminum core 1.
Having such a makeup, the copper cladding layer in the conventional Cu/Al composite wire generally forms from 5 to 20% of the cross-sectional area of the composite wire. The aluminum core material is a high-purity aluminum alloy whose purity is 99.9% or more or an Al-Fe alloy containing 0.9 to 2.5 percent iron (see Japanese Patent Unexamined Publication No. 53-110082). Such Cu/Al composite wire had a conductivity of 60 to 70% IACS (International Annealed Copper Standard), a specific gravity of 3.0 to 3.95, a tensile strength of 10 to 20 kgf/mm.sup.2 (soft-drawn) and 20 to 35 kgf/mm.sup.2 (hard-drawn) (American Society for Testing Materials (ASTM) B566-88).
As described above, the coverage of the copper cladding layer is from 5 to 20% at the cross-sectional area. Often the copper cladding layer breaks to expose the aluminum core when the composite wire is soldered to printed circuits or boards, thus causing defective connections. Also, adjusting, stranding, and handling of the wire in thin diameter during the intermediate process may cause damage to the copper cladding layer, leading to exposure of the aluminum core, which the becomes corroded or breaks, etc.
On the other hand, if the copper clad takes up more than 20% of the total cross-sectional area of the wire, the strength of the core against the copper clad layer is impaired in the case where the core material is a high-purity aluminum alloy material. In this case, the aluminum core may break easily when the wire is drawn. Consequently, drawing of wire to obtain small sizes is difficult. Also, when high-purity aluminum alloy is used, even if the copper forms less than 20% of the cross-sectional area of the wire, the tensile strength is small (35 kgf/mm.sup.2 or less). Such a wire exhibits insufficient strength which is fatal as a wiring conductor when drawn to be 0.15 mm.phi. or less in diameter. In addition, the conventional Cu/Al composite wire has exhibited conspicuous inferiority in corrosion resistance to salt water at the end faces thereof compared with ordinary copper wire. Further, although various types of copper-cladding materials are used while utilizing the electromagnetic shielding property of copper, no study has been made on the Cu/Al composite wire.